The use of Bragg reflective gratings in optical fibre waveguides for waveguide dispersion compensation in optical fibre transmission systems has been described in GB 2 161 612 B. The dispersion compensator uses a chirped Bragg reflective grating in an optical waveguide connected to one port of a 3-port circulator or of a 2.times.2 3 dB directional coupler. Light is directed into one port of the circulator or 3 dB coupler from which it is launched into the waveguide with the chirped Bragg grating. The reflected light is launched back into the circulator or (3 dB coupler) to emerge from a different port. Conventional transmission fibre typically exhibits a dispersion zero at a wavelength in the region 1.3 .mu.m, and therefore, for transmissions in the region of 1.5 .mu.m, this fibre exhibits anomalous dispersion (i.e. longer wavelengths in the region of 1.5 .mu.m suffer a longer transit time through the fibre than shorter wavelengths in this region). Accordingly, to achieve a measure of dispersion compensation, the Bragg grating should delay shorter wavelengths more than longer wavelengths, this by reflecting the longer wavelengths at distances closer to the circulator (or 3 dB coupler) than those at which the shorter wavelengths are reflected. In other words the large pitch end of the chirped Bragg grating reflector should face the circulator (or 3 dB coupler). The Bragg reflective grating based dispersion compensator that employs a 3-port circulator is depicted in FIG. 1A, while its counterpart that employs a 2.times.2 fused fibre 3 dB directional coupler is depicted in FIG. 1B. Referring to FIG. 1A, a 3-port circulator 10 has ports 10a, 10b, and 10c. Ports 10a and 10c constitute respectively the input and output ports of the dispersion equaliser. Port 10b is optically coupled with a chirped Bragg reflective grating 12. The dispersion equaliser of FIG. 1B is distinguished from that of FIG. 1A in that the place of the circulator 10 of the dispersion equaliser of FIG. 1A is taken by a 2.times.2 fused fibre 3 dB coupler 14 provided with ports 14a, 14b, 14c and 14d. Ports 14a and 14c constitute respectively the input and output ports of the dispersion equaliser. Port 14c is optically coupled with the waveguide 11, while port 14d is not used.
The dispersion equaliser of FIG. 1A may be preferred to that of FIG. 1B because the circulator 10 does not introduce the same intrinsic loss as the 3 dB coupler 14. When power is launched into output port 14a of coupler 14, there is a minimum of 3 dB loss because half of this power is directed to output port 14d, instead of to output port 14b, and so is wasted. Similarly, a further minimum of 3 dB loss occurs because half of the power reflected by the Bragg grating, instead of being direct into the output port 14c, is directed out of the input port 14a, and so is wasted. On the other hand a 3-port circulator is liable to be much more costly than a 3 dB fused fibre coupler.
The problem of the 6 dB extra loss involved in the use of the 3 dB coupler has been addressed by M J Guy et al in a report entitled, `Low-loss fibre Bragg grating transmission filter based on a fibre polarisation splitter`, Electronics Letters, Sep. 1, 1994, Volume 30, Number 18 pages 1512-3. In particular this report describes a way of avoiding the 6 dB loss penalty by a method involving the use of an optical fibre polarisation beam-splitter instead of a 3 dB coupler, the use of a second Bragg reflector optically coupled with the fourth port of the polarisation beam-splitter, and the use of two quarter wave birefringence elements. This arrangement is depicted in FIG. 2. Referring to FIG. 2, a polarisation beam-splitter 20 has ports 20a, 20b, 20c and 20d. Ports 20a and 20c constitute respectively the input and output ports of the dispersion equaliser. Light of any arbitrary state of polarisation (SOP) applied to input port 20a is resolved into orthogonal linearly polarised components respectively emerging by way of ports 20b and 20d. From these ports the light is launched via respective quarter-wave birefringence elements 21 and 21' into respective fibres 11 and 11' provided with identical chirped Bragg reflectors 12 and 12' optically substantially equidistant from the polarisation beam-splitter 20. The quarter-wave birefringence elements 21 and 21' are aligned such that the linearly polarised light from each of the ports 20b and 20d is launched into their respective fibre 11 and 11' as circularly polarised light. The light that is reflected by the respective gratings 12 and 12' is also circularly polarised, but now has the opposite handedness. Accordingly, after transmission back through birefringence element 21, the light being launched back into polarisation beam-splitter 20 by way of port 20b from fibre 11 is polarised orthogonally with respect to the SOP of the light that was launched from port 20b into fibre 11. Therefore this reflected light passes back through the polarisation beam-splitter 20 to emerge exclusively by way of port 20c. Similarly, after transmission back through birefringence element 21', the light launched back into polarisation beam-splitter 20 by way of port 20d from fibre 11' is polarised orthogonally with respect to the SOP of the light that was launched from port 20d into fibre 11'. Therefore this reflected light also passes back through the polarisation beam-splitter 20 to emerge exclusively by way of port 20c.
The foregoing analysis has failed to take any account of any birefringence that may be present in either of the fibres 11 and 11'. If there is any such birefringence, this will make the `optical pitch`, by which term is meant the product of the physical pitch with the effective refractive index, of any pair of Bragg reflecting elements of a Bragg grating different for the two different principal SOPs. The physical pitch is the same in both instances, but the presence of birefringence means that their effective refractive indices are necessarily different. This means that any particular part of the chirped grating will reflect a waveband centred on one wavelength for one of the principal SOPs, and will reflect a waveband centred on a slightly different wavelength for the other principal SOP. In other words, in the presence of birefringence in the waveguide in which the Bragg reflection grating is created, the reflection produced by that grating produces polarisation mode dispersion (PMD).
Polarisation maintaining optical waveguide has fast and slow axes defining, together with the waveguide axis, an orthogonal set of axes. The two planes that contain the waveguide axis and either the fast axis or the slow axis define principal planes, these having the property that light of a single wavelength plane polarised in a principal plane propagates in the waveguide with a single velocity. The velocity is greater for light propagating plane polarised in the principal plane containing the fast axis than for that plane polarised in the other principal plane (containing the slow axis). For light propagating with any other state of polarisation (SOP), this light is resolved into its principal plane components which, on account of their propagation with different velocities, give rise to the phenomenon of polarisation mode dispersion (PMD).
An idea of the scale of the problem of PMD in the Bragg reflection grating type dispersion compensators of an optical transmission system can be arrived at by considering a 10 Gbit/s system. The pulses in such a system are 100 ps wide. If there are a number of dispersion compensators in cascade along the transmission path, their PMD will be cumulative, and so the PMD of each may typically be required to be less than 1.0 ps. In a typical optical fibre, light will travel about 1 mm in 5 ps, and so, if Bragg grating of a dispersion compensator is written in birefringent fibre, the point along the grating at which one principal plane component of light of any chosen freespace wavelength is reflected should, taking into account the folded path, be separated by less than 0.1 mm from that at which the other principal plane component is reflected. If the grating is 2 meters long, is centred on a free-space wavelength of 1.5 .mu.m, and covers a (freespace) spectral range of 30 nm, then the rate of change of pitch is 15 pm per mm. Under these conditions the proportional change in reflected wavelength over a 0.1 mm distance along the grating is 1 part in 10.sup.6. Therefore, for the Bragg grating not to induce more than 1.0 ps PMD, the proportional difference between the effective refractive indices of the two principal planes should not exceed 1 part in 10.sup.6.
As a practical matter, to make an optical fibre with a birefringence as low as this is not always an easy task because unintentionally introduced departures from perfect circular symmetry are very liable to introduce significant birefringence. With the exercise of due care, it is possible to make conventional transmission type optical fibre with birefringence lower than this. An example of a very low birefringence fibre of this sort may typically have a PMD in the region of 0.0005 ps/m, which corresponds to a birefringence index difference of about 1 part in 10.sup.7. However optical fibre designed for high photosensitivity suitable for having Bragg gratings written into it typically has a core/cladding index difference (.DELTA.n) greater than that of conventional transmission fibre (typically .DELTA.n.apprxeq.0.01 instead of .DELTA.n.apprxeq.0.005), and the greater index difference is found to exacerbate the non-circularity problem. Moreover there can be good reason for wanting to use fibre with an even higher .DELTA.n, typically fibre for which .DELTA.n.apprxeq.0.05. A reason for this is that increasing .DELTA.n provides a `blue-shift` to the short wavelength loss exhibited by chirped Bragg reflective gratings. If .DELTA.n is large enough, the blue-shift may be sufficient to take this loss, or at least a significant proportion of it, outside the operative wavelength band of the transmission system in which the dispersion compensator is to be located.
A fibre whose .DELTA.n is about 0.05 is typically found to exhibit a birefringence of about 0.05 ps/m, making it a medium birefringence fibre with a beat length of about 10 cm. This corresponds to a birefringence index difference of about 1 part in 10.sup.5.
According to a theory developed by R Ulrich, `Fiber-Optic Rotation Sensors & Related Technologies, Proceedings of the First International Conference` page 67, the phase shift per meter .beta. introduced by bending an optical fibre of radius r into a curve of radius R.sub.c is given by EQU .beta.=0.565.times.10.sup.6 (r/R.sub.c) rads/m
at a free-space wavelength .lambda.=1.5 .mu.m. From this it can be shown that, for 125 .mu.m diameter fibre, a bend radius of about 6 mm or less is required to cancel the birefringence of a fibre with a beat length of 10 cm, and so such fibre is polarisation maintaining fibre under normal operating conditions.
It is now seen that, though the dispersion equaliser of FIG. 2 does offer a way around the 6 dB loss penalty, it does this in a manner that ensures that any birefringence in fibres 11 and 11' is certain to introduce PMD into the system because light is propagating in these fibres as circularly polarised light.